1. Field of the Invention
The present invention relates to a rubber modified styrenic resin composition which is excellent in balance between the appearance and the impact strength of molded articles and which has high gloss, and it also relates to a method for preparing the same.
2. Description of the Related Art
Molded articles of ABS resins are excellent in balance between appearance and impact strength, and so they are widely used in domestic appliances, electronic equipments and the like. In recent years, a tendency toward the cost reduction of these products has occurred, and in consequence, rubber modified styrenic resins are often used in place of the ABS resins. Furthermore, these rubber modified styrenic resins become used to manufacture molded articles having a large size, a complex shape and a small wall thickness. Under such situations, a rubber modified styrenic resin having high gloss, excellent moldability and workability, and high impact strength is desired.
The rubber modified styrenic resin is a styrenic resin in which rubber particles are dispersed, but the particle size of the dispersed rubber particles has a large influence on the quality of products. The smaller the particle size of the dispersed rubber particles, the better the gloss of molded articles that is obtained therefrom. The size of the rubber particles (an average particle size; the same shall apply hereinafter) in the rubber modified styrenic resin is usually in the range of from about 1.0 to about 5.0 .mu.m. Nowadays, in order to improve the gloss of the molded articles, a resin has been developed in which the rubber particles of 1.0 .mu.m or less are dispersed. Usually, in the rubber modified styrenic resin in which the rubber particles having a particle size of 1.0 .mu.m or less are dispersed, impact strength is noticeably low. Therefore, it is difficult to improve the gloss, while the impact strength of the molded articles is maintained. In order to solve these problems, there have been suggested some methods which comprise the step of blending a rubber modified styrenic resin containing dispersed rubber particles of 1.0 .mu.m or less with another rubber modified styrenic resin containing dispersed rubber particles of 1.0 .mu.m or more in Japanese Patent Publication No. 41467/1971, Japanese Patent Application Laid-open Nos. 1519/1984 and 241053/1988, and U.S. Pat. No. 4,146,589. However, these methods still have some drawbacks such the insufficient gloss and the poor balance between the impact strength and the gloss.
It is well known that when a styrene monomer in which a styrene-butadiene block copolymer having a high styrene content is dissolved as a rubber component is polymerized, a polystyrene can be obtained in which rubber particles having a particle size of 0.5 .mu.m or less and a capsule particle morphology are dispersed [e.g., Angew. Makromol. Chem., 58/59, pp. 175-198 (1977)]. Japanese Patent Publication No. 18594/1973 and Japanese Patent Application Laid-open Nos. 500497/1986, 48317/1988 and 74209/1989 have suggested some methods for preparing rubber modified styrenic resins which are very excellent in gloss and transparency of molded articles by the utilization of the above-mentioned well-known technique. According to these methods, the surface gloss and transparency of the molded articles can be indeed remarkably improved in contrast to the conventional rubber modified styrenic resins. However, the impact strength is still insufficient, though its improvement has been attempted.
In U.S. Pat. No. 4,493,922, Japanese Patent Application Laid-open No. 112646/1988 and the like, it is attempted that a rubber modified styrenic resin, in which rubber particles having a capsule particle morphology formed by using the above-mentioned styrene-butadiene copolymer rubber are dispersed, is blended with a usual rubber modified styrene resin containing a small amount of dispersed rubber particles having a cell or coil particle morphology so that the particle size distribution curve of the dispersed rubber particles may have two peaks (so that the distribution curve may have two maximum values) and so as to improve the impact strength, while the gloss of molded articles obtained therefrom is maintained at a high level. In these methods, the impact strength can be indeed improved, but the gloss is still insufficient and even the impact strength is still unsatisfactory. Therefore, there is a problem that a polydimethylsiloxane or the like must be added to the resin.